In recent years, compact and wearable head-mounted display devices to be worn on the head or glasses have been proposed. Known examples of such a head-mounted display device include the one that enlarges light of an image (hereinafter, called “image light”) displayed on a compact display element fixed to the temple by a lens and guides the image light to the eyes of the user by an eyepiece optical system (a light guide prism) to display the image light as an enlarged virtual image in a field of view of a user. (Refer to Patent Literatures 1 and 2, for example.) Using the compact and lightweight light guide prism allows the entire appearance of the head-mounted display device to be compact and lightweight without interrupting sightlines considerably. Such a head-mounted display device may be used as a wearable device.
However, the smaller the light guide prism becomes, the more likely it is the light (hereinafter, called “side surface reflected irregular light”) reflected by, for example, inner side surfaces of the light guide prism is produced. When projected into the eyes, the side surface reflected irregular light tends to cause ghost and flare. The ghost, in particular, appears closer to the image present in the field of view as the cross-sectional dimension of the light guide prism becomes smaller, thereby giving severe discomfort feeing to the user. In view of the above, Patent Literature 2 provides the light guide prism, on the side surfaces thereof, with V-shaped grooves having a large tilt angle relative to the side surfaces, in order to remove the unwanted side surface reflected irregular light in the light guide prism.